Slide projection lenses are designed to be well-suited for a variety of applications. They are usually fast lenses (large aperture, small f-number) to maximize image brightness on the screen. This is advantageous in situations where ambient light cannot be completely eliminated. However, larger apertures have two disadvantages compared to smaller ones: (1) less depth of field; and, (2) reduced Modulation Transfer Function (MTF) due to increased aberrations. Less depth of field causes center to edge sharpness fall-off with open-mounted slides, and requires more accurate and more frequent focusing (whether manual focus or autofocus). Reduced MTF is manifested as poorer sharpness over the entire projected image.
Incorporation of an iris diaphragm into a projection lens design allows optimal aperture choice.
When the surroundings are completely dark, the lens can be stopped down (smaller aperture) and improved sharpness, better center to edge sharpness uniformity, and more lenient focus tolerances can be achieved. If there is stray ambient light, however, the lens can be completely opened up, so that the image brightness overpowers the veiling flare due to ambient illumination, and thereby avoids excessive contrast loss.
However, virtually no presently marketed projection lenses incorporate iris diaphragms due to cost considerations. This means that projectionists do not have the control over the lens characteristics necessary to obtain the best possible results in a variety of circumstances.